Muscle contraction is switched "on" and "off" in response to external chemical or electrical signals. Within each muscle cell activation is controlled by calcium ions and regulatory proteins: at high levels of calcium the muscle is "on", at lower levels the muscle is "off". This calcium switch consists of the proteins tropomyosin and troponin. Using X-ray crystallography together with electron microscopy I propose to determine the detailed three-dimensional structure of tropomyosin, visualize its interactions with troponin in the crystal lattice, and prepare crystals of troponin and/or its subfragments for high resolution structure determination. This information will allow us to better understand how these proteins control contraction. Motions of tropomyosin have been postulated to play a critical role in the regulatory process. The flexibility of tropomyosin in the crystals will be examined by analyzing the diffuse X-ray scattering in conjunction with crystallographic studies at various temperatures. These results will allow us to determine the kinds of motions and conformations which tropomyosin can adopt during the regulatory process. Although tropomyosin was first discovered in muscle cells, we have recently recognized that this protein plays an important role in the cytoskeleton of many cells. Moreover, tropomyosin-like proteins have been found at the surface of certain pathogenic bacteria, and appear to be critical factors in determining their virulence. Attempts will be made to characterize the molecular structure of one of these proteins from streptococcal bacteria. Knowledge of how this "M protein" works could be significant in developing ways to control streptococcal diseases.